Interpretive Summary: Measurement of the body's nitrogen content is the most direct measurement of body protein. We have used a statistical technique, called 'Monte Carlo' simulation to aid in the design of a new system for the measurement of nitrogen in the human body. This approach allowed us to quickly test several different designs without having to actually built prototypes. This approach proved significant saving in expense and time. An optimum design, based on precision, length of time to perform the measurement, and minimum dose to the subject, was developed.

Technical Abstract:
The prompt- and delayed-gamma neutron activation techniques have been used for the non-invasive measurement of human body composition. In recent years, neutron irradiators have used only transuranic isotopic sources (238PuBe, 241AmBe, 252Cf). However, in today’s security-minded environment, the use of alternate neutron sources may provide some advantages. We have examined several designs for an irradiator that would use a high-output, miniature D-T neutron generator (MF Physics). The use of this type of neutron source will lessen the storage, security, and transport issues associated with continuous-output isotopic neutron sources. To determine the scientific impact of this decision, we have performed Monte Carlo simulations (MCNP-4B2; Los Alamos National Laboratory) to aid in the design of the irradiator system, evaluating shielding materials, collimation, and source-to-subject distance, for the measurement of total body nitrogen (TBN). Based on internal flux distributions within the simulated body region of a subject, several design options were identified. The final design will be selected based on the optimization of precision, dose, and exposure time